Evan Harting can't pinpoint the exact time he realized Maryland Deathfest — the annual four-day celebration of metal and its many subgenres he created with his friend Ryan Taylor — was bigger than he imagined. Those brief, surreal moments tend to come when he's wandering the sea of sweat and hair, and least expecting it.

"During certain times of the night when I'm walking around, I'm just like, 'Wow, this is really happening,'" he said.

Harting, a 27-year-old from Parkville, has reason to be surprised. Maryland Deathfest began modestly, with Harting and Taylor, 30, two friends from Parkville High School, taking a gamble that they could put on a better metal festival than one they had seen in Ohio. The duo had booked tiny local metal shows before, but nothing close to a multi-day onslaught. Now, Harting sounds like a veteran booker.

"The process is about the same every year, with just a little bit of tweaking from what we've learned from the year before and feedback," he said. "We start on the next one the week after the last one is over."

The planning continues to pay off. Judging from tickets sales, this year's Deathfest will likely see its largest turnout ever.

It kicks off Thursday in Sonar's main room, with bands such as Die Pigeon Die and Eyehategod. On Friday, it expands to its three-headed monster — two outdoor stages and one inside. Harting expects roughly 4,000 metal-heads on Friday and Saturday respectively, and "a little bit more" on Sunday.

He continues to be amazed by the festival's continued growth.

"In the beginning, I was not doing this to turn it into a crazy metal festival," Harting said. "I just take it year by year, and it seems to grow. I never expected it to happen that way."

Another surprising Deathfest element is how far fans will travel for it. Metal enthusiasts from within the United States have to surrender bragging rights to fans who had to pack passports.

"A lot of people come from Australia, New Zealand and Japan," he said. "I don't think it gets much farther than that."

But out of the 60-plus acts playing this year's Deathfest, the biggest draw, according to Harting, could be Electric Wizard, the esoteric English doom-metal quartet that hasn't played America in more than a decade.

"They don't do a whole lot, especially in the States, so it's going to be a rare appearance," he said.

Landing a band as rare as Electric Wizard is just the latest example of Deathfest's growth and reach. Harting says that after this year's festival, he and Taylor will decide whether it's time to leave Sonar (where it has been since 2006) for a larger, all-outdoors venue.

"It's something we've definitely been thinking about in the back of our minds the past couple years," he said.

Trey Williams, drummer for Upper Marlboro metal group Dying Fetus, will play his second Deathfest on Thursday. He says Deathfest fans appreciate Harting and Taylor keeping the festival "exclusive" by booking bands not heard on 98 Rock.

"They've got their niche and it's working," Williams said. "There's a risk of taking the next step up and you possibly alienate your fans. It being such a small festival, it makes people clamor to get there."

If Maryland Deathfest leaves Sonar, there will likely be opportunities to book bigger, more mainstream metal acts. But toss around some names — say, Metallica or Iron Maiden — and Harting gets uncomfortable.

"We're booking somewhat bigger bands but they're still underground," he said. "I never want to lose that feel. … At least for now, I would rather keep it the way it is. You don't see Slipknot on [the bill] and the people that go appreciate that."

Maryland Deathfest takes place Thursday-Sunday at Sonar, 407 E. Saratoga St. Godflesh, Morbid Angel and many more will perform. Doors open on Thursday at 4 p.m. Ticket prices vary. Go to marylanddeathfest.com.